The present invention relates to an ultrarefined arabinogalactan polysaccharide gum of a defined molecular weight range and to the method for its preparation. The invention is particularly concerned with an arabinogalactan polysaccharide material with a molecular weight distribution within about 6,000 and 2,500,000 which makes essentially no contribution to the osmolality of aqueous solutions in which it is a solute.
Partially refined arabinogalactan gums have been available commercially for a number of years. This polysaccharide occurs widely in many plant and bacteria species. It is found in significant quantities in the trees of the genus Larix, particularly in the lower portion of Larix occidentalis Nuttall (western larch). The characterization, production, and uses of larch arabinogalactan gums is discussed by M. F. Adams and B. V. Ettling in Industrial Gums, 2 ed., R. L. Whistler and J. N. BeMiller Eds., pp 415-427, Academic Press, New York (1973). In some old growth trees the arabinogalactan is present in the butt log in such great quantities as cause ring shake (circular splits) and render the log much less valuable for lumber manufacture. This same wood, when comminuted, provides a ready source of arabinogalactan when extracted with warm water. Extraction is normally carried out in countercurrent manner with water at a temperature below about 70.degree. C., as is described in U.S. Pat. No. 3,337,526 to Adams.
Arabinogalactan is a polymer of arabinose and galactose in varying ratios which are combined to form molecules with a general ball-like configuration. There is a wide range of molecular sizes varying from monomolecular and low molecular weight homopolymers to macromolecules with molecular weights well over 2,500,000. Molecular weight distribution tends to be bimodal with peaks at about 50,000 and 90,000.
A number of other chemical species, normally considered undesirable, are also extracted along with the arabinogalactan. These include especially the tannin-like lignans, aromadendrins, other simple and complex hydroxyaromatic compounds, and terpenes. At least some of these compounds appear to be hydrogen bonded to the arabinogalactan. As a first step in refining, heat treatment alone, such as by spray or drum drying the crude extract, will serve to partially break these hydrogen bonds. Another method of breaking the hydrogen bonds is by digestion of an aqueous solution in the presence of a polar compound such as methanol. A preferred method of refining is by treatment with an active form of MgO at pH 9-10 by digestion at elevated temperature, as taught in U.S. Pat. No. 3,325,473 to Herrick et al. This causes the phenolic and iron containing compounds to precipitate, leaving a clear yellow solution of arabinogalactan. A product made by this method is available as Stractan 2, from Champion International Corp., Libby, Mo. Despite the aforenoted refining, the presence of residual color indicates that not all of the phenolic materials are removed. This is confirmed by measurement of ultraviolet transmission characteristics and tannic acid equivalent.
One procedure for obtaining a more highly refined arabinogalactan has been published by L. Corash in Red Cell Metabolism, E. Beuttler, ed. Vol. 16, Chap. 8, pp 99-106, Churchill Livingstone, London (1986). Here the MgO treated arabinogalactan referred to above is further refined by sequential treatment first with a cation then an anion exchange resin. This procedure is slow and cumbersome but has been used to produce a fairly pure arabinogalactan in small quantities sufficient for research purposes.
An important use of the more highly refined arabinogalactan, such as that described by Corash, is as a component of solutions for density gradient separation of biological materials such as blood cells, viruses, and organelles. These solutions must in general be isotonic and physiologically harmless to the material being separated. One such procedure using arabinogalactan was described by Corash et al. in the following article: Separation of erythrocytes according to age on a simplified density gradient. Journal of Laboratory and Clinical Medicine, 84 (1): 147-151 (1974).
While the material purified by the Corash procedure has been very useful, it has not been completely satisfactory since it still has appreciable color, taste and odor. It has now been discovered that this is due to the appreciable amount of retained low molecular weight species of arabinogalactan and other materials. In addition to the aforenoted deficiencies, the Corash product still contributes significantly to the osmolality of resulting solutions. The most desirable product would have little or none of the potentially toxic aromatics and make little or no contribution to solution osmolality. Stated differently, the arabinogalactan could be used for its independent contribution to solution density without introducing interacting factors, such as osmolality upsets, that required compensation. Up to the present time no such product has existed.